Clean up pointer use:

* Use unique_ptr for Actions instead of manual memory management
 * Use unique_ptr in KeywordDatabase
 * Use unique_ptr instead of shared_ptr for file formats
 * Don't pass shared_ptr to Reader/Writer, use references instead
Also
 * Switch to C++17 so we can use map::try_emplace

src/render/value/image.cpp

@@ -32,7 +32,7 @@ void ImageValueViewer::draw(RotatedDC& dc) {
     // load from file
     if (!value().filename.empty()) {
       try {
-        InputStreamP image_file = getLocalPackage().openIn(value().filename);
+        auto image_file = getLocalPackage().openIn(value().filename);
         if (image_load_file(image, *image_file)) {
           image.Rescale(w, h);
         }